The long-term objective of these studies is to develop and characterize a mouse model of malignant melanoma that can be used to investigate tumor development, metastasis, and treatment in the primary autochthonous host as well as in syngeneic transplant recipients. We have developed a method for the induction of spontaneously metastasizing primary melanomas in C57BL/6 mice. The melanomas obtained vary greatly in latent period prior to the development of the primary tumor, as well as in tumorigenicity, metastatic ability, and stability of the original phenotype. In the present application we propose to investigate spontaneous metastasis in a variety of recently induced mouse melanomas and then to evaluate selected host and tumor cell characteristics that may determine metastatic ability during tumor development. During the past year, we have evaluated the phenotypic stability of the carcinogen-induced JB/MS and JB/RH melanomas. Transplantation of the MS melanoma over a 3-year period has maintained the original melanotic phenotype. In contrast, the RH melanoma became primarily amelanotic after the second transplantation in vivo and has remained amelanotic over a 2-year period. Election microscopy has confirmed the presence of melanosomes in the amelanotic RH tumors. Propagation in culture has resulted in a decrease in tumorigenicity for both the JB/MS and JB/RH melanomas. High passages of the RH melanoma require at least 10 times as many cells to produce tumors in C57BL/6 mice as do low passages of the RH cell line. The MS melanoma decreases in tumorigenicity even more rapidly in culture. However, early passages (e.g., 1 to 3 subcultures) do produce melanotic tumors which metastasize to the draining lymph nodes of nude mice. Our results suggest that, for studies of newly-induced melanomas, it is essential to: (1) limit the number of tumor passages in vivo and in vitro; (2) choose methods of propagation that preserve the original phenotype; and (3) distinguish those properties produced by technical manipulation from those produced by true tumor progression. (S)